Conformational changes of the recombinant extracellular domain of E‐cadherin upon calcium binding

The cell‐adhesion protein E‐cadherin/uvomorulin exhibits a calcium‐dependent homoassociation. The effect of Ca 2+ on the extracellular fragment of E‐cadherin was studied using the recombinant protein expressed in the baculovirus expression system. The recombinant and native fragment of E‐cadherin were found to be similar by many biochemical criteria [Herrenknecht, K. & Kemler, R. (1993) J. Cell Sci. 17 , 147–154]. A large and reversible conformational transition was observed upon Ca 2+ depletion. A change from a rod‐like structure, 22 nm in length, to a more globular assembly of the five subdomains became evident by electron‐microscopical analysis. In the presence of Ca 2+ , the circular dichroic spectra indicated predominantly β‐structure but a more negative ellipticity was observed in the absence of Ca 2+ . The intrinsic tryptophan fluorescence decreased by 12% upon Ca 2+ depletion. Both effects were used for calcium titrations which indicated calcium binding to several sites with average K d values of 45–150 μM. Cleavage of the protein fragment by trypsin occurred only at low Ca 2+ concentrations and from the calcium‐dependence of cleavage rates, a K d value of 24 μM was derived. The major site of cleavage was identified by partial sequencing to be located between the two putative calcium‐binding sites in the third subdomain from the N‐terminus. In agreement with earlier results with the native fragment, the recombinant protein did not associate in the presence or absence of Ca 2+ . We suggest the calcium‐dependent homoassociation therefore depends on additional effects connected with the cell surface association of E‐cadherin.